a) Field of the Invention
The present invention belongs to the field of pulsed lasers, and particularly relates to a self-seeded pulsed fiber laser allowing spectrum and wavelength selection from pulse to pulse.
b) Brief Description of Prior Art
Pulsed laser sources are currently of considerable interest, particularity for communication-related applications. It is usually desirable to produce a high peak power from a pulsed laser. Three techniques are currently used for that purpose: Q-switching, mode-locking, and gated cascade amplification.
The Q-switching method consist of switching from a high-loss to a low-loss condition in a laser cavity. A Q-switched laser system typically comprises a gain medium, pumped by a diode laser or other external source, and a mirror on each side thereof to generate the laser oscillation. The switching between a high-loss and low-loss condition is generally obtained with an acousto-optic modulator. Before switching to the low-loss condition, the gain medium is fully inverted and presents its maximum gain. Reverting rapidly to a low-loss cavity enables the build-up of a powerful pulse in the laser. The resulting peak power is fairly large, but the spectrum is often composed of several longitudinal modes and the repetition rate is generally low. Moreover, the pulsewidth is not directly adjustable, and it varies with the pumping rate, the repetition rate and the cavity optical length. Another drawback is a "jitter" of the output beam, that is substantial variations of the delay between the moment when the pulse is triggered and the launching of the pulse.
Mode-locking is another technique to obtain high peak power and short pulses, by synchronizing most of the longitudinal modes of the laser cavity with an internal modulator. The driving frequency of the modulator corresponds to the round-trip time of the cavity and has to be precisely tuned. Therefore, the repetition rate of a mode-locked laser is fixed as well as the pulsewidth, since they are determined by the physics of the cavity.
In order to have control over both the repetition rate and the pulsewidth, one can use a gated cascade amplification. A low power laser diode is first pulsed with the right repetition rate and pulsewidth and acts as a seed for a series of amplifiers, which increase the pulses power. This has the advantage of separating the pulse generation from the amplification process, both the spectral and temporal quality of the pulse then depending on the laser diode source. However, to change the wavelength, to eliminate the longitudinal modes or to change the spectral shape of the source light, one has to change the laser diode or use multiple lasers. In addition, each amplifier in the chain generates it's own noise which is then amplified by the following amplifier, rapidly saturating the pulse power. A solution to this problem is to gate each or some of the amplifiers with a switch, to limit the self-saturation of the chain. The switches must however be activated synchronously to the passage of the pulse, which makes for a very complex system.
There is therefore a need for a pulsed fiber laser with easy control over both the repetition rate and the pulsewidth, while still internally allowing for wavelength tuning and optical spectral shaping.